One typical but non-limiting example of sheets of the above-mentioned kind consists of sheet metal intended to be used for manufacture of stampings, pressings or drawn articles where the initial operation in such manufacture is to form a plurality of flat blanks from the sheet, the blanks being of a shape, for example circular, such that not all of the sheet can be used. Metal, in common with other raw materials, is relatively expensive but its scrap value (if any) is much less. Scrap therefore represents a loss which is only partly recoverable, and even in the case of metals which are generally capable of being recycled for further use, the recyling process itself involves the employment of energy and therefore the use of fuel and possibly other natural resources.
The technology of pressing flat blanks from relatively thin metal sheet has advanced to a stage at which it is now possible to leave, between holes left in the sheet after pressing, webs of metal which at their thinnest point are only wafer thin. Nevertheless, in the case for example of blanks in the form of discs or rings, there must inevitably be a considerable amount of metal in each sheet that is wasted between the holes purely by virtue of the shape of the blanks.
Wastage also occurs at the edges of the sheet due to the fact that, whilst for certain non-rectangular shapes of blank, such as discs or rings, the most economical pattern to adopt is to blank the metal in rows which are usually staggered transversely, there will always occur at one end of each row an area of the sheet which is unusable and which, in the case of staggered rows in which alternate rows are identical, will be of a size approximately one-half of that required for making a row of blanks. Since sheet metal from which stampings, pressings or drawn articles are made (especially where such articles are relatively small and of simple shape) is mostly supplied in coils and edges of the coils are approximately straight, this wastage at the end of each row is usually accepted.
However, in many applications, especially those in which the sheet material has to undergo some preliminary operation or operations before being subjected to the stamping operation, it is not convenient to form the stampings from a long strip or web of the sheet material drawn direct from the coil; in such cases the strip or web must be cut into individual sheets for the preliminary operation or operations. As indicated above, it is in connection with such individual sheets that the present invention is conceived. Examples of such preliminary operations include treatment to remove irregularities at the edges or surface irregularities; plating; lacquering; and printing. For example, tin plate sheet for use in making components for food or beverage cans is for many applications pre-lacquered or printed or both.
Cutting individual rectangular sheets from the continuous strip drawn from the coils does however result in there being in each sheet, not only two side edges giving rise to wastage as described above, but also a front and a rear edge each of which causes wastage between the blanks. It has been a common practice, in order to overcome this disadvantage, to cut the individual sheets along a zigzag line such as to bisect the material lying between the sites of two adjacent transversely-staggered rows of blanks on the strip or web. In this way a castellated edge, usually referred to as a scrolled edge, is formed. This eliminates the wastage that would otherwise be caused by the formation of individual sheets.
Both because materials are becoming more precious and because they are becoming ever more expensive, thinner and thinner materials are being used for many products. This is especially true in the metal container industry, particularly in the case of so-called open-top cans, where relatively thin metal is now used for making can ends and can bodies.
In addition, modern container-making machinery, in common with other kinds of machinery used in the manufacture of pressed, stamped or drawn articles of which components for container are but one example, is in general capable of higher speeds than its predecessors. This applies also to machines such as printing machines which may print the sheet material before the articles or blanks of the appropriate shape are formed from it. Use of higher speeds and thinner sheet in general call for improved accuracy in manufacture. On the other hand thinner sheet is less susceptible to high accuracy, for example because of its greater flexibility and liability to undergo damage to the side edges, a common fault found in coils of tinplate as delivered to the user.
In conventional apparatus used for feeding sheet material to a stamping or drawing press (such as a blanking and cupping press) or to a printing machine, it is usual to employ the side edges of the sheet to guide it accurately in its predetermined orientation into the press. Side edge damage and lateral whip give rise to problems with this method, limiting both the accuracy of lateral or transverse location and also the speed at which the sheet can be conveyed. A further serious problem is that sheet material from different coils may not always be of the same width, since the tolerances on the nominal width which it is possible to obtain, particularly with thin sheet steel and other metals made in high speed rolling mills, are greater than is generally acceptable for high speed conveying of the sheet to a high speed precision machine such as a modern blanking and cupping press.
Various solutions to these problems have been proposed, and some successfully employed, such as the use of side lays which travel along with metal sheet and converge gradually into engagement with the side edges to guide it accurately into a high-speed printer.